The present invention relates to wireless communication, and more particularly to minimizing power losses during wireless communication.
Backscatter modulation is a common communication technique in many RFID applications. Backscatter modulation can be used in a passive RFID tag to send data back to an RFID reader without using a transmitter. Instead, communication is accomplished by repeatedly shunting a coil in the RFID tag through a transistor, the shunting causes slight fluctuations in the RFID reader's carrier amplitude. As the secondary winding is momentarily shunted, the primary winding experiences a momentary voltage drop. The RFID reader can detect this voltage drop and interpret that data as a bit. Bits can be strung together by systematically shunting the secondary winding. The data bits can be encoded by a variety of encoding algorithms that may affect error recovery, cost of implementation, bandwidth, or synchronization capability.
Backscatter modulation and variations thereof have also been used outside the RFID context, for example in communication between an inductive power supply and a remote device in a wireless power transfer system. U.S. patent application Ser. No. 11/855,710 filed by David W. Baarman on Sep. 14, 2008, entitled “System and Method for Inductively Charging a Battery” and U.S. Patent Application No. 61/019,411 filed by David W. Baarman on Jan. 7, 2008 entitled “Inductive Power Supply with Duty Cycle Control” disclose shunting a resistor in a remote device to communicate with an inductive power supply, and are both herein incorporated by reference.
There are a number of challenges with backscatter modulation and similar techniques that occur within inductive power supply systems that are not as problematic in traditional RFID systems. In an inductive power supply system, the amount of power delivered to the remote device may vary. For example, a mobile phone in standby mode will likely draw significantly less power from an inductive power supply than the mobile phone would draw during a telephone call. As the amount of power drawn by or delivered to the secondary increases, the power losses associated with communicating increase. Further, power losses are often amplified because the communication resistor for a remote device is typically selected during manufacture to account for worse case scenarios. This additional reliability often comes at the price of additional power losses during communication, even in normal or optimal conditions. The nature of a wirelessly powered load can become dynamic with the variances of coupling associated with spatial movement which is complicated by a wide array of device requirements from light to heavy loads. This dynamic can change the power requirements of the system, adjustments to the system as described herein can help account for the changing power requirements.